1. Field of the Invention
This invention relates to an ion-selective sensor having an electron-conducting electrode and a membrane for determining the selectivity and to a method of producing the sensor.
2. Description of the Prior Art
Ion-selective sensors serve to determine the electro-chemical activity in electrolytes and can be used directly or indirectly for numerous quantitative measurements. They are therefore enjoying increasing popularity in numerous fields of physics, chemistry and medicine.
Ion-selective sensors are available in various different types of construction. The best-known is the classical construction of the glass electrode, more particularly as a pH electrode for determining hydrogen ion activity but also for measurement of other types of ions, having glass of different composition. These glass electrodes have proved to be reliable and are characterized by good stability and a long service life. One disadvantage of these electrodes lies in the fact that the properties of selectivity of the ion-selective glass available at present do not fully satisfy the user's requirements.
In latter years, specific membrane sensors have been developed in order to select particular types of ions. The contact electrode of the ligand electrolytic system is, separated from the medium to be measured or analyzed by means of a membrane made from an organic polymer. The membrane contains largely organic carrier molecules or ion-selective ligands as its effective component in a polymerized base material of the membrane. An example of an ion carrier of this type is valinomycin which has a very high selectivity for potassium ions. Very high selectivity coefficients can also be achieved by other means with certain materials. Polyvinylchloride (PVC) is often used as the base material for these membranes. In order to optimize the properties it may be necessary to introduce additionally into the membrane phase softeners for example and/or lypophilic anions.
In the case of sensors having these plastics membranes however, difficulties of a different types arise, more particularly with respect to sealing between the membrane and the sensor casing and because of the limited stability of the shape of the membrane material. The mechanical yielding thereof can for example lead to faults owing to fluctuations of the pressure in the measuring system. In the case of certain sensor constructions moreover, the permeability of the membrane to gases and water vapor may introduce measurement errors. The service life of sensors utilizing a membrane has up to now also been considerably lower than that of sensors using glass electrodes.
The present invention aims to make available an ion-selective electrode having the very high selectivity of membrane electrodes and also having a large reliability factor, a long service life and other advantageous characteristics of the glass electrode.